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New Method Helps Remove Weather Effects from Air Pollution Data

In light of the impacts caused by the current COVID-19 pandemic, satellite measurements demonstrated that northern Italy and China have experienced reduced levels of nitrogen dioxide (NO2).

Traffic density, wind and air stratification influence the pollution with the air pollutant nitrogen dioxide, according to the conclusion of a TROPOS study commissioned by the LfULG, for which data from 29 stations in Saxony were evaluated—including Dresden. Image Credit: Burkhard Lehmann, Saxon State Office for Environment, Agriculture and Geology.

Traffic density in Germany is the most significant factor. But according to a study performed by the Leibniz Institute for Tropospheric Research (TROPOS), the weather also impacts the level of NO2. TROPOS also assessed the effect of weather conditions on NO2 concentrations in Saxony earlier in 2015 to 2018, as part of the Saxon State Office for Environment, Agriculture and Geology (LfULG).

The study also demonstrated that the height of the lowest layer in the atmosphere and the speed of the wind are the most significant factors that establish the amount of pollutants that can build up at the local level.

To find out the effect of numerous weather factors on air quality, the researchers employed a statistical technique through which meteorological changes can be mathematically eliminated from long-term measurements. Different emissions and the effect of the weather can cause changes in air quality and quite strongly in certain cases.

Until now, it has been hard to estimate, for example, the role of weather and its effect on the actual air quality. What are the contributions of legal measures such as diesel driving bans and low emission zones? With the technique employed, this will become easier in the days to come.

The irritant gas NO2 assaults the mucous membrane of the respiratory tract boosts the impact of other air pollutants and leads to inflammatory reactions as an oxidant. Since NO2 is a precursor substance, it can even contribute to the development of particulate matter.

The EU has set limit values to safeguard the population: for NO2, a yearly average value of 40 μg/m3 is acceptable. If these limit values are not fulfilled, measures should be taken to safeguard the health of the general population. For instance, in 2018 and 2019, numerous measures were implemented in Germany, spanning from driving bans for older diesel vehicles (for example, in Stuttgart) to a reduced number of lanes (for example, in Leipzig).

To assess the effectiveness of these measures, it may be useful to find out the actual effect of weather conditions. Therefore, TROPOS was commissioned by LfULG to perform research on the effect of weather factors on the levels of NO2. For this purpose, LfULG offered its measurement information from the Saxon air quality measurement network and meteorological data.

This allowed the scientists to assess the data from 29 stations located in Saxony across a four-year period, representing a cross-section of air pollution—from stations located at traffic centers to rural and urban background stations, as well as stations located on the edge of the Erzgebirge mountains.

The scientists also estimated the height of the lowest layer in the atmosphere and integrated the data, obtained from traffic counting stations based in Dresden and Leipzig, into the study. For statistical modeling, the scientists used a technique from the area of machine learning. The application of this technique in the field of air quality was initially published by British scientists back in 2009.

In this manner, the study effectively showed that the traffic density existing at all traffic stations is purely responsible for the concentrations of NO2. But a couple of weather parameters was also found to have a major impact on the concentrations of NO2—that is, the height of the supposed mixing layer and wind speed. The former is a meteorological parameter that denotes the height to which the lowest atmospheric layer, where the emissions combine, extends.

It was also shown that high humidity can also reduce the concentration of nitrogen dioxide, which could be due to the fact that the pollutants deposit more strongly on moist surfaces. However, the exact causes are still unclear.

Dr Dominik van Pinxteren, Scientist, Atmospheric Chemistry Department, Leibniz-Institute for Tropospheric Research

In addition, the statistical analysis has allowed the scientists to eliminate the effect of weather from the time series of concentrations of pollutants—modified for the weather, the levels of nitrogen oxides (NOx) reduced by a total of 10 µg/m3, between 2015 and 2018 on average across all traffic stations located in Saxony.

But in rural and urban regions, as well as on the edge of the Erzgebirge, the concentrations of NOx tend to stay at the same level. Although air quality improved to a certain extent in the recent past, there are good scientific debates for additional reduction of air pollution.

To some extent, this also holds good to early conclusions reached from the current corona crisis—to determine the extent of the actual impact of the preliminary limitations on air quality, the effect of the weather would have to be statistically eliminated in a longer sequence of measurements.

As such, analyses performed for the Leipzig region are presently ongoing at TROPOS and so does a Europe-wide analysis of the EU research infrastructure intended for short-lived atmospheric constituents, like clouds, aerosol, and trace gases (ACTRIS)—the German contribution managed by TROPOS.


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